Frequency divider



Aug. 16, 1949, 'OR M 'N 2,479,180

FREQUENCY DIVIDER Filed Oct. '8, 1945 sheets-sheet 1 POWER AMDLIFI ER s .120 q 6d 8 6 a U0) INVENTOR.

O 'ERNST NORRMA-N 2 Sheets-Shet 2 Filed Oct. 8, 1945 wum a o Gmmu JOE-r200 oom omw cow 0% com 0mm M M M R M w m N M r @3241 .w T 5530mm. 5 N R E Patented Aug. 16, 1949 UNITED STATES OFFICE FREQUENCY DIVIDER 1 Ernst Norrman, Williams Bay, Wis, assignor to The George W. Borg corporation, Chicago, 111., a corporation of Delaware 1 Application October 8, 1945," Serial No. 620,915

8 Claims. (01. 250-36) This application is a continuation in part of my prior application Serial Number 477,237, filed February 26, 1943 and now Patent Number 2,410,389.

The present invention relates in general to frequency conversion apparatus, and more in particular to socalled frequency dividers which are employedto obtain a desired low frequency output from a high frequency input. The object of the invention is to produce a new and im proved apparatus of this character.

. The invention is useful, for example, in generating standard frequency alternating current for running the constant speed motor of a watch timing device. In such a generator a source of constant frequency alternating current is required. A crystal oscillator a substantially constant frequency and accordingly is very desirable for this purpose, but the frequency is far too high for use in running a motor. However, by the use of a suitable frequency divider the high crystal oscillator frequency (in the neighborhood of 90,000 cycles per second) can be reduced to a sub-multiple frequency of 100 cycles per second or some other desired low frequency, and by suitable amplification of the low frequency output of. the frequency divider the required low frequency current for running the motor may be obtained.

Apparatus of the foregoing character, com:- prising a crystal oscillator, frequency divider, and amplifier, may be referred to as a constant frequency generator, and has been used extensively in connection with watch timing apparatus. In

this known generator the frequency divider coml required. For these reasons the cost of a standard frequency generator using multivibrators is rather high.

A specific object of the present inventiom therefore, is a frequency divider using single tube oscillators instead of multivibrators. Another object is a frequency divider having a larger division at stage than has been consideredpractioable in frequency dividers using multivibrators. I

.A well known single tubexosclilator is the socalled Hartley oscillator, employing a tuned circuit and'a grid circuit inductively coupled thereto. Thestandard Hartley oscillator, how: evenis entirely unsuited for use as a frequency divider, for reasons which need not be gone into in detail. Foronething, the standard Hartley oscillator is not susceptible to reliable control at a sub multiple frequency :by alternating currents derived from a crystal oscillator. Another diliculty arises from the fact that if it is attempted to control a Hartley oscillator at a sub multiple frequency from a similar oscillator the controlled oscillator tends to operate as an amplifier at the controlfrequenoy. I

In thissituation, Ihave discovered that with suitable coupling arrangements which I have also devised a-somewhat different single tube oscillator can be made to, operate very satisfactorily as a frequency divider," The new oscillator difiersfrom the standard Hartley oscillator in several respects, as will becxplained fully hereinafter in describing its construction and operation. One

feature which is important and which may be mentioned at this time is the wave shape of the oscillator grid voltage. I have discovered that for satisfactory operation the positive half of the Wave must be decidedly limited in amplitude, with a characteristic flat top.

Another feature of the invention relates to the solution of the problem involved in causing an oscillator to divide by an odd or an even number; and to the discovery I have made that for the best results an oscillator designed for odd division .has to be different from an oscillator designed .for even division; The first step in the solution ,of this problem was the discovery that excellent results could be obtained byinsertin'g a condenser and leakresistor in the grid circuit of an oscillater intended for odd division and by omitting these parts in thecase of an oscillator intended for even division. Further investigation in the course of which a larger number of different circuits were tried resulted inthe discovery that the ratio of plate to grid coupling in the oscillator transformer is of importance in this connection.

' Pursuing this clue, and after. extensive tests of different transformer ratios, I have found that an oscillator for odd division the ratio of the ference in the transformer windings the oscillators for odd and even division may be the same.

of tests made on a 90 cycle oscillator such as is shown in Fig. 2.

Referring to Fig. 1, the complete equipment therein shown is a standard frequency generator which may be used, for example, to supply alternating current for running a small motor at constant speed. The principal component parts of the generator are a crystal oscillator, a threestage frequency divider, and a power amplifier.

The crystal oscillator and power amplifier are of known construction. The invention relates to the combination of these elements with the frequency divider and more in particular to the frequency divider per so, as previously indicated.

The frequency of the crystal oscillator is assumed to be 90,000 cycles per second, asindicated in the drawing. The three oscillators which comprise the three-stage frequency divider divide, respectively, by 10, and 9, and oscillate at frequencies of 9,000, 900 and 100 cycles per second, respectively. This arrangement, giving an output frequency of 100 cycles per second, is satisfactory for the intended purpose, although it will be understood that the invention is not limited to the employment of any specific frequencies. By selecting a different oscillator frequency, or by adjusting one or more of the oscillators at the frequency divider to divide differently, or both, various other output frequencies may be obtained.

Describing the equipment more in detail, the reference character 10 indicates a suitable space discharge device, which may be a type 6J7 pentode. This tube functions as a combined oscillator and amplifier tube. The control grid and the screen grid are coupled to the crystal, as shown, by means of condensers. The screen grid functions as the plate or anode in this circuit, being supplied with anode potential through the resistor ll, which may have a value of 500 M ohms. The control grid is connected to ground through the resistor I9, which may have a value of one megohm. There is a reactance coil 20, value about millihenrys, which is connected in series with the crystal l6, and the oscillator is tuned to the exact frequency by means of the condenser I8. The control and screen grids function also in the amplifier. The suppressor grid is connected to the cathode as shown, and the plate is connected to the plus B lead through the resistor 2|, which may have a value of 500 M ohms.

The first oscillator stage of the frequency divider comprises a space discharge device II, which may be a type 6J5 triode. Explaining the oscillator circuits briefly, the cathode is grounded, and the plate is connected to the plus B lead through the resistor 25, the value of which may be'50 M ohms. There is a tuned circuit comprising the inductance 29 and the condenser 28 which is connected between the plate and ground by means of the condenser 21. This condenser preferably has a value of .001 MF and is shunted by the 100 M ohm resistor 26. The inductance 29 is 4 one winding of a transformer the other winding 30 of which is connected to the control grid of the tube through the 10 M ohm resistor 24. The ratio of primary turns to secondary turns is preferably about 4 to l, as indicated.

In order to control the oscillator H from the preceding stage the control grid of tube II is coupled to the plate circuit of tube ID by means of the resistor 23 and the condenser 22. The resistor 23 preferably has a value of 150 M ohms, while the capacity of the condenser 22 may be MMF.

The second oscillator stage of the frequency divider comprises the space discharge device 12, which may also be a type 6J5 triode. This oscillator is similar to the first stage oscillator except for different values of some of the circuit ele ments. Thus the coupling condenser 3| preferably has a value of .01 mi. The ratio of turns in transformer winding 32 to the turns in the secondary winding 33 is 2 to l. The resistors 34 and 35 preferably have values of 50 M ohms and one megohm, respectively.

The third oscillator stage of the frequency divider comprises the space discharge device l3, which may be another type SJ 5 triode. The coupling condenser 40 preferably has a value of .1 mi. The turn ratio between the transformer windings 4| and 42 is 2 to l, and the values of the resistors 44 and 45 are preferably 50 M ohms and 500 M ohms, respectively. There is a condenser 46 (capacity .05 mf.) in the grid circuit, shunted by the leak resistor 41 which preferably has a value of 100 M ohms.

The power amplifier comprises the tubes l4 and I5, which maybe of any suitable type, and includes the output transformer 50. The conductors 5i and 52 constitute the output circuit, which may lead to a small synchronous motor. The amplifier is of the push-pull type and the grids of the tubes are controlled from the third oscillator stage of the frequency divider, as indicated in the drawing. In this connection it will be noted that the oscillator transformer has a third winding 43. Windings 42 and 43 together have the same number of turns as winding 4|, whereby equal and opposite voltages to ground are available for application to the grids of tubes I4 and i5. The resistor 55 is the usual cathode biasing resistor.

It will be understood that if the frequency divider has an even number of stages, four for example, two type 6F8 double triodes may be used instead of four type SJ 5 triodes.

The apparatus having been described, the operation of the complete standard frequency generator will now be "explained.

The operation of the crystal oscillator and amplifier stage it] is well known and need not be described in detail. Suflice it to say that by the operation of the crystal oscillator section of the tube l0 fluctuating currents having a frequency of 90,000 cycles per second are set up in the plate circuit of the amplifier section of the tube, whereby fluctuating voltages of the same frequency are made available at the plate for application over the control circuit to the grid of the tube I l of the first oscillator stage of the frequency divider.:

and resistor 23 to the grid of tube t.

The econd sta e. lcsc ldtci" 1:2 9 the tt uth y diyider oscillate at s treeuehoy o etc cycle Pe cond- I is controlled at t1 t at frequency by vo ta develop d te ter uned circu t ate-2:9 o 'the stag eto eud tra smitted to the rid o tube 1: throu h the resistor 35. V I

The third st e sci la r is of t e Ireuueucy d ider osci a s a a re ue c o tilt cy les er second and controll d at th t tree or .hy vo t es d v lop d across he tuned ,c rm 3:32 o t e co d ta oscil tor and rdnsm ttcd irciiit oi tube it by t e ct the co denser it b w y of res to .45 a d g id condenser .46 to the grid of tube 173. r

The p h-p l. amplifier H 5 emn iiies the 10.0 c cle output oi he thi d st ge .os etcr.

iound i de irab e 1cc e conden e o '11 lower y in o der t th powe tr us. r ed- Otherw e t o h h e th a e apnea. acros th tuned. circu t end it to ver difiiciilt to p ope y roport on. he other circuit nts- It w ll he n ted tha the c densers uch as '31 have d fi rent ca acity valu depe ding on t e fre uency of the se store o which they helcn I Conden e 3;! at capac ty of .91 con e s :21 ha a c pecity of .001 while condenser 40 has capacit ct ,1 m "llhese va ues a e not c tica v i-it t is im rtant to v lu s at the described imitin e i ect secu ed.

said ou put m sin e ual nd cement vo t:

ages. to ground whi h are d vel ped in the Windin M a d w ndin s .4 and 4,3 f t e-oscilla or tran o er-c The e volta es cont ol. the operation of th a plifi in manner, w th, the result that alter a n current havin a tre qu ncy of .100 cyc s p r se o d transmitt d over the output circuit 51752,

Th f regoing a mo e. or less enera descrintion of the op ration... Att tion will now be directed more pa ti larly o the fea ur s oi th oscillators in the frequency d vider hereinbefore mentioned and other features which are ble or essential t s tisia tory per m nce.

The tuning of the oscillators may first be consi ered. The oscil ator 12, for examp e, QSCilr' la e u der control at a irequency of 90 cycles. per second. It I dy be tuned to that frequency, but. is n i h tuned to a somewhat lower frequen y. ih tuni accomplished by s ec in a c nden r 3.6 02. the proper capacity and by addin a sm ll. trimmer condenser as indicated, if required. The frequency to which t os illator should be tun d tor the best results is determined by ascertaining the upper and bitter limits of the natural :or uncontrolled free quency which the oscillator may have and be susceptible to control at 90,0 cycles per second, A frequency is then selectedwhich is, higher than the lower frequency limit by approx mately -one= third of the total range between the upper and lower limits, and the oscillator then tuned to this frequency.

To illustrate the foregoing, I have found that if other factors are properly taken care of, as will be explained presently, the oscillator I12 may have a natural frequency ranging from the lower limit of about 820 cyclesper second to the upper limit of about 915 cycles per second and still be susceptible to control at 900 cycles. per second trorn the preceding 9,0D0-vcycle oscillator. This a range of 95 cycles. One-third of 9t5wcycles is approximately 32 cycles, which when added to 82;). cycles gives 852 cycles per second as the proper frequency to which the oscillator should be tuned. V

The reason for tuning to the low side of the range is two-fold. The operative natural flee quency range extends farther below the op rate th equenc of- 9 0 cycles. er second than it does. c eit, nd the chan e may he e ct o o c time such hehecsin a onden rap i o va iation in oi the resistors, eudt inc as the-m ureli esh ter o he o ci lse ot The co ense the ccuc iueecudenser hrou h wh ch newer dehseted: from the. late it has be n .icuhd that hen the condenser- 61. for e amp e, s m de of lo enough cenacity sub a a l imi th pow r n t rred rte-the associated tun d ci cuit it has at considerable ttect on he tun n o t e un d ci cu t, and any chan e whi h nu ht occur in the capacity of t e cond n would ch n et e oscil ator tre uuenc app eciab y- It is d irab e-to ieliniinetc many changeable fee-tors as possible, and us cordin ly t e condense i shunted by the h h r is a resi tor 31. W e t is shunt is used the c pa ity of con enser 31 con heese by as much s 2.0% without ausi fe ure ot the osci lator t id rope l it s. ler shunt resista ce 26 is connected a ound thec ndenser 21 at the oscillator H.

The tunedcirc t the e e al osc lato s h u d e ch h a rather hi h ind ctance Th f c t a the o ator r d vo e should ave a we' e sh ch r c ze by a low tim pli-tude flat-topped positive half wave preic s be n m n ioned. At th oscillator 1 the required a s a i s r by m ns oi-the hig istance 4 i he rid ci cuit.- When the rid swing o t e i starts to draw current, but the resistanc 34 is so high relative to the internal resistance of the tube (cathode to grid} that the major part of the total voltage in the grid, circuit is developed across theresistor 35 and he o e ial o e d r tive t the cathode is limited to a low value. Similar re sistors are used in the grid circuits of the other oscillators in the frequency divider. This 9J1, important feature, since it impossible to prop, er cont o he o c l rs vc the desired su mu l f equ n s unless th re i o ar prose ent in the grid, circuits v c The ratio oi the control voltage to the scillator d v a is e o he cn c ht im o ance, Considering the oscillator 12 again! thegrid volt: a e i j t v l a ener t in t e se o. try ndin 33 du o i sv cou ing With'thec. o, winding 32, It is the coupling between these windings, one in the plate circuit and the othe'r in the grid circuit, that makes the tube oscillate. By the term control voltage E mean the voltage that is transmitted fromthe preceding stage and applied to the grid of tube l2't0- cause'it to oscil latesat exactly the desired frequency. This volt age is transmitted over .a circuit to ground which includes the resistor 35., the resistor 34 and: the winding 33, the latter having such a low resist ance that it. may be neglected. The grid .of the tube 12 being connected to the junction of'res'is: tors. 3d and 34., these resistors, -operatic :asra welt amiss 7 age -divider' or potentiometer and-the ratio of resistance 35-to resistance 34 determines what proportion of the total voltage available will be effective at the grid of the tube.

Now I have found that for the best results the control voltage should be approximately 15% of-the grid voltage. The desired ration is secured by a proper proportioningof the factors which affect it, but mainly by selecting a resistor 35 having the proper resistance value.-

-Instating that the best results are secured when the control voltage is 15% of the grid voltage, I'-mean that with this ratio of controlvoltage to gridvoltage the oscillator may lee-tuned to anyfrequency within 'a maximum frequency range and still be susceptible to control at the desired frequency'of 900 cycles per second, and that if the ratio is changed substantially in either direction the frequency range is correspondingly narrowed. This may be illustrated by the results of -tests on the 900-cycle oscillator I25 It was previously mentioned that this oscillator may have a natural or uncontrolled frequency ranging' from 820 cycles per second to 915 cycles per second, but it will be understood now that it has this large frequency range only when the control voltage bears'the optimum relation to the grid voltage. I have foundthat if the control voltage is raised to 30% of the grid voltage the natural frequency range within which the oscillator may be'tuned becomes approximately 825 cycles to 880 cycles, while if the control voltageis decreased to l /2% of the grid voltage the frequency range becomes approximately 870 cycles to 920 cycles. These values show a considerable narrowing of the range and indicate the importance of maintaining the ratio of control voltage to grid voltage within fairly close limits and as near to the best ratio as is practicable. The same ratio is used at all the oscillators of the frequency divider.

Attention has already been directed to the fact that a condenser is used in the grid circuit at the oscillator I3, which divides by 9. I have discovered that in the case of an oscillator which divides by an odd number the insertion of a condenser of the proper capacity in the grid circuit approximately doubles the natural frequency range within which. the oscillator may be tuned. In the case of an oscillator which divides by an even number, however, the use of a grid condenser is of no avail and actually decreases the range. Accordingly, at any stage where 'odd division is required the grid condcnseris used, but is omitted at other stages. -'Referring now'to Fig. 2, the standard frequency generator therein shown comprises a crystal oscillator and a four-stage frequency divider. The power amplifier which will usually be required has been omitted but it will be understood that a suitable amplifier can be driven from the output conductor H5 at the last stage of the frequency divider, or if desired a push-pull amplifier can be coupled to the final oscillator by the circuit arrangement shown in Fig. 1.

As indicated in the drawing, the crystal oscillator operates at a frequency of 81,000 cycles per second. The four oscillators which form the fourstage frequency divider operate at frequencies of 16,200, 2,700, 450, and 90 cycles per second respectively, dividing by 5, 6, 6, and 5 respectively. As in the case of the frequency divider shown in Fig. 1, practically any desired output frequency may be obtained by selecting a different oscillator frequency, or by changing the rate of division at one or more-stages,- or by a combination of these methods. 4 7

a The crystal oscillator comprises the crystal IUI and-the space discharge device III), which may be'a type 6J7 pentode operating as a combined oscillator and amplifier tube. The circuit arrangement is generally similar to the circuit of the-crystal oscillator shown in Fig. 1 and will not need to be described in detail.

1 The first oscillator stage .fof the frequency divider comprises a space discharge device III, which may be a type 6J5 tube, and the associated circuit elements shown.- The cathode is grounded, and the anode is connected to the plus B lead through the resistor I I6, which may have a value of 50 M ohms. The tuned circuit comprising the inductance H8 and the condenser I2I is connected between the anode and ground by means ofthe condenser III, which preferably has a value of about .0015 mi. This condenser may be shunted'bya resistor if desired for reasons explained in connection with Fig. 1, but the shunt is not strictly-necessary and in the circuit of Fig. 2 it has been omitted. The inductance H8 i's'onewinding of a transformer the other wind? ing IIS of which is connected to the. control grid cf'the tube through the resistor I23, which may be M ohms. The ratio of the number of turns in winding lit to the number of turns in winding II 9 may be.3 to-2, as indicated in the drawing, and should not differ greatly from this ratio, as willbe explained more fully hereinafter.

The oscillator I I I is coupled to the crystal oscillator by means of the condenser I24 and the resistor I25. The condenser may have a capacity of mmf. while the resistor may be 500 M ohms.

The second-oscillator stage of the frequency divider comprises the space discharge device I I2, which may be another 6J5 tube, together with the other circuit elements'shown. The essential difference between the second stage oscillator H2 and the first stage oscillator III resides in the different. transformer ratios which are employed. In the oscillator III, which divides by the odd integer 5, a 3 to 2 ratio is used, as already explained, whereas inthe oscillator II2, which divides by the even integer fi, a ratio more than twice as great is used. Excellent results have been obtained when the ratio of the number of turns in the plate Winding I2I to the number of turns in the grid winding I28 153.5 to l. The ratio may be somewhat lower and may be as high as 4 to 1.

It will be understood of course that the coupling condenser I26 should have a higher capacity than the corresponding condenser I I 1 in the first stage, due to the lower operating frequency of the second stage. Condenser I26 may have, for example, a capacity of .005 mf. The control resistor I30 .and the grid resistor I29 may be the same as the corresponding resistors in the first stage, that'is, these resistors may be 500 M ohms and '75 M ohms, respectively. It is found, however, that somewhat better results are obtained if rcsistors of other values are used, the values being selected so as to compensate roughly for the lower grid voltage which results from the 3.5 to 1 transformer ratio, and thus avoid disturbing to any great extent the best ratio of the control voltage to the grid voltage. It was explained in connection with Fig. 1 that the control voltage should beabout 15% of the grid voltage, although the relation is not critical. In view of the foregoing it'is preferable to use at each even division stage, control and grid resistors so selected that the difi fer-encebetween: their values is considerably greaterthan the:' difference between; the: values of the corresponding resistors. at theodd stages, Excellenti'results have been-obtainedwith a control resistor Itiiof- 650rM-.- ohms and a grid resistor I29 of 5 M 1 ohms;

The, third stageoscillator l-l3 may be'the same as 'the secondstage oscillator H2, except that it is'tunedto a=loWer frequency; The coupling corrdenser l 3l may havea'capacity of .025 mf.

The fourth stage oscillator ll4 divides by 5' and accordingly is similar to the first stage oscillator I'H- except that it is tuned'to a different operating frequency. A coupling condenser 132 having a capacity of .10 mi. maybeusedi v While-four ordinary-triodes have been shown inthe: drawing, it will be understoodthat type 6SN-7 tubes or the equivalent may be used By using these double triodes the number oftubes' required for afour-stage frequency divider is reduced to two tubes. f

The operation will bereadily understoodfrom the explanation previously given in connection with Fig. 1. The crystal. oscillator operates at a frequency of 81,000-cycles per secondanddrives the first stage oscillator III of the frequency divider, which divides by 5 and operates at a frequency of 16,200 cycles per second. The oscil-. lator H ldrives the second stage oscillator 2 which divides by 6 and operates at a frequency of 2,700 cyclesper second. The third stage os-. cillator H3, dividing by 6' and operating at a frequency of 450 cycles per second-isdriVen-bytheoscillator ll 2 and in turn drives thefinalstage oscillator M which divides by- :5 and operates at a frequency of,90'cycles per second.- The output conductor I I5 may be used to control any suitable power amplifier, as previously mentioned.

Reference may now be made: to Fig. 3,- which= shows the selective action of two'dilferent trans.- former ratios as affecting odd and even division. This chart or graph, constructed from actualtest of a 90 cycle oscillator, shows that with a trans;- f'ormer ratio of 3- to 2 the oscillator strongly favors an odd division, while with atransformer ratio of'3.5 to 1 the oscillator shows a strong-tendency to operate at a frequencywhich is the result-of. an even division.

The test was made by driving an oscillator such as oscillator I M; Fig. 2, from a variable frequencyoscillator adapted to supply driving potentials having about the same voltage as the control volt age obtained from oscillator H3" and having. a variable frequency ranging from about 200. cycles. per second up to about 1,000:cyclesper second.

The heavy. horizontal lines; on the chart which are labe1'ed"Odd3:2 at the right-show the frequency ranges for oddand even division when; the oscillator is designed forodd-divisionby using a transformer with a 3 to 2 ratio.

The heavy horizontallines which arerlabeled Even-35:1 at the rightshow the corresponding" frequency ranges whentheoscillatoris de signed for even division by using a transformer; with a 3.5 to 1 ratio. 3 a

In the oscillator intended for odd di-vision the. control and grid resistors such as l-33-andl 3g4. preferably have values of 500- M-ohms and 75; M respectively, while in the-evendivision-oscillator the corresponding resistors. preferably have values of" 650 M' ohmsand150- M,- ohmsa. as-

control and: grid resistors are the same in: both 10 oscillators? although. the discrimination is --not so pronounced 1mma k-ingz'afitest, on theoscillator designed for oddsdivisiong fpn example, the-variable frequency drivingv oscillatpr? isad-justedtodel-iver driving potentials; having alow frequency such as 250 cycles per; second; and is. connected to the. oscil latoritorbeitestedi The frequency of the driving voltageds then raised -graduallyand; at the. same time: the output, frequency.--' of the oscillator is compared irriwell knownimannerwith thezdriving or input frequency by meansof a. cathode ray oscilloscopa-so. -that;the rate of; division can-be observed. Since. the oscillator bein tested. is tuned-a for operation; at. 90 cycles per: second, it divide at a rate which will enable it to op emte-ataa frequency; most: nearlyrapproximating QBycycles second; and willdncrease. its rate of divisiprr asrthe input frequency is raised.

Thus whenthe frequency of the. driving volt-. age is. 27.0; cycles pergsecondthe. oscillator di-, vides 3,. when: the frequency of the driving voltage isfififl cycles pen-secondthe oscillator divides-blst hand so on.. This+is shown clearly-by, the digits 3; Y 4; 5,1 etc: which i are associated with theheavy lines on the chart. Theseilines-also shoywnby their lengths the frequency ranges for stableoperation atthe differentvrates of division; Fordivisionby 5;,f-cr example; the oscillator has a range of, input or controlrvoltages of: over 100 cycles-per second, 'I herange for division by 7. is; evengreaten. The, rangefordivision by: 6, on the-oth'er. hand, is. less=than- 50 cycles: per secondiand for divisiombyA isstill-less.

- At this; pointzit-should be. explained that an oscillator suohlas disclosedherein; when oscillating. under control, at a sub multiple frequency, displays a certaintendency to maintain its; rate Qf-l division= notwithstanding a change in; input frequency to a; value outside the stable.- range corresponding. to such rate: of division. tendency accounts for? the. facts that the heavy r ange;linesi-d o not@ join each other but? are sepa rated"- by spaces corresponding ttoifrequency ranges; inlwhrchi the oscillator is unstable or will give either an; odd or even. division. For: example, if the inpuievoltagc; is 400. 0YC18SP81'1S6C0Dd the oscillator will divide by- 5 and-it willacontinue-to, divide: by 5* as the frequency of the. inputvoltage i i- Q a edhn -l-the f e n y r ac es th low erlimi-tt of the, divisionv by 6' range, whereupon the; oscillator: will start: to'. divi'daby- 6.; If the frequency of the control voltageis-now decreased;

- the-oscillator does not atoncestart-to divideby by noting the frequencies: at. which; th'er oscillator chan es; its. rate: of division.

Attentipn may new be directed to thei-upper'set of range lines, which weredeterminedby testing a -QQ cycle:- cseiilatorin which the? plateto: grid ransfo m cwnl 'na ra ie w l be seen: that, for; divisiontby- 6- the. oscillator has. a range of controtveltages of more than. 12.5;cycles. It also has a very wide range for divisionbwi. r iri on b 5i e e he ra o ly about 50.- cycles, and-for division by '7 the range isionllt' Belittlemorethan- 25 cycles.

Comparison: of the two-sets of; range lines de.- monstrates clearlythatam oscillator in which the: coupling ratio is 3 to: 2' favors anodd; division;v

whereas an oscillator with a 3.5 to 1 coupling ratio favors an even division. It will be apparent therefore that a frequency divider in which the oscillator stages for odd division have transformers with a coupling ratio of about 3 to 2 and in which the oscillator stages for even division have a coupling ratio of about 3.5 ml is inherently more stable, that is, less susceptible to trouble due to changes in tuning occurring over a period of time, than a frequency divider in which the coupling ratios at the odd and even stages are the same.

From what has been said already it will be understood that the coupling ratios to be used are not critical. An oscillator having a transformer with a coupling ratio of 3.5 to 1 (best ratio for even division) will divide by 5 or other odd integers but prefers to divide by an even integer. Asthe coupling ratio is changed to lower ratios such as 3 to 1, 2 to 1, etc. the range for odd division improves and excellent results are obtainedwith a ratio of 1 to 1. The best ratio appears to be about 3 to 2, in the oscillator shown herein. A similar situation exists as regards even divisions; An oscillator with a transformer having a coupling ratio of 3 to 2 (best for odd divi- 'sion) will divide by 6 or other even integer, but'the range for even division is improved by increasing the ratio. The maximum improvement appears to result with a ratio of 3.5 to 1, although 4 to 1 gives excellent results also.

The fact that an oscillator having a coupling ratio of 3 to 2 favors odd division which an oscillator having a coupling ratio of 3.5 to 1 favors even division appears to indicate that for odd division an oscillator requires a higher grid voltage than for even division. As referred to here the grid voltage is the voltage applied to the grid of oscillator III, for example, through the resistor I23 by the oscillator transformer com- 1 prising windings H8 and H9. It will be obvious that a higher voltage will be developed in the grid circuit with atransformer ratio of 3 to 2 than with a transformer ratio of 3.5 to 1. Tests have been made therefore to seeif equivalent results could not be obtained by adjusting the value of resistor I23, using a resistor of low value for odd division and a resistor of considerably higher value for even division, but without success. Changes in the value of resistor I23 affect the value of the applied control voltage and also affect the ratio of control voltage to grid voltage, so that it is impossible to obtain any appreciable results as regards grid voltage without such an unfavorable effect on other factors that no net advantage results. 7

In the description certain types of tubes have been specified, and actual values have been given for various parts such as condensers and resistors, but it will be understood that this has been done merely by way of example and to assist in the understanding of certain concrete embodiments of the invention and the principles which govern their operation.

The invention having been described, that which is believed to be new and for which the protection of Letters Patent is desired will be pointed out in the appended claims.

I claim:

' 1. In a frequency divider, first and second oscillators each comprising a grid controlled space discharge device, a tuned plate circuit including the primary winding of a transformer, and a grid circuit including the secondary winding of such transformer, said frequency divider also ineluding a control circuit over which periodic voltages are transmitted to control the first oscillator, and a second control circuit over which periodic voltages are transmitted by the first oscillator to control the second oscillator, the plate circuits in said oscillators being so tuned that one oscillator operates at a frequency which is an odd sub-multiple of its control frequency while the other oscillator operates at a frequency which is an even sub-multiple of its control frequency, the transformer in the oscillator which operates at the odd sub-multiple frequency having a relatively low ratio of primary to secondary turns to cause such oscillator to favor an odd division, and the transformer in the oscillator which operates at the even sub-multiple frequency having a substantially greater ratio of primary to secondary turns to thereby cause such oscillator to favor an even division.

2. A frequency divider as claimed in claim 1, wherein the ratio of primary to secondary turns in the transformer of the oscillator which operates at the even sub-multiple frequency is at least double the ratio of the primary to secondary turns in the transformer of the other oscillator.

3. A frequency divider as claimed in claim 1, wherein the ratio of primary to secondary turns in the transformer of the oscillator which onerates at the odd sub-multiple frequency is not less than about 1 to 1 nor more than about 2 to 1 and the ratio of primary to secondary turns in the transformer of the other oscillator is not less than about 3 to 1 nor more than about 4.5 to 1.

4. A frequency divider as claimed in claim 1, wherein the ratio of primary to secondary turns in the transformer of the oscillator which operates at the odd sub-multiple frequency is approximately 3 to 2 While the ratio of primary to secondary turns in the transformer of the other oscillator is approximately 3.5 to 1.

5. A frequency divider as claimed in claim 1, wherein at each oscillator there is a resistor included in the control circuit and another resistor in the grid circuit, the values of said resistors being so selected that the ratio of control voltage to grid voltage is approximately the same at both oscillators notwithstanding the difference in transformer ratios.

6. A frequency divider as claimed in claim 1, wherein each oscillator has a resistor in its control circuit and another resistor in its grid circuit, the said resistors having resistances such that they compensate at least in part for the effect which the different transformer ratios at the two oscillators has on the ratio of control voltage to grid voltage at such oscillators.

'7. In a frequency divider, a plurality of oscillators, a space discharge device in each oscillator having cathode grid and plate elements, a tuned plate circuit in each oscillator, a grid circuit in each oscillator inductively coupled to .the associated plate circuit, a control circuit for controlling the first oscillator from a high frequency source of current, and coupling circuits over which each oscillator except the first is controlled from the preceding oscillator, said plate circuits being so tuned that each of one or more of said oscillators operates at a frequency which is an odd sub-multiple of its control frequency while each of the remaining oscillators operates at a frequency which is an even sub-multiple of its control frequency, and transformers constituting the inductive couplings in said oscillators having windings so designed that the ratio 13 of plate to grid coupling at each oscillator which operates at an even sub-multiple frequency is at least twice the ratio of plate to grid coupling at the oscillators which operate at odd sub-multiple frequencies.

8. The method of improving the stability of an oscillator for operation as a frequency divider, said oscillator having a, tuned plate circuit including the primary Winding of a transformer and a grid circuit including the secondary winding of such transformer, which consists in using a low or high transformer ratio depending on whether the oscillator is to divide by an odd or even integer, the ratio of primary to secondary turns for odd division being not less than about 15 REFERENCES CITED The following references are of record in the file of this patent:

, UNITED STATES PATENTS Number Name Date 2,036,495 Pohontsch Apr. 7, 1936 2,236,532 Gibbs Apr. 1, 1941 2,410,389 Norrman Oct. 29, 1946 

